The present invention relates generally to wireless broadcast transmissions and more particularly to a method for optimizing the delivery of multicast and broadcast transmission packets, especially in an 802.11 network.
Unless otherwise defined herein, the terms in this specification should be interpreted as defined, or as customarily used, in the Institute of Electrical and Electronics Engineers (IEEE) 802.11 and 802.11e specifications. The IEEE 802.11 and IEEE 802.11e specifications are hereby incorporated by reference in their entirety. The current draft standard refers to the current draft supplement to the 802.11e specification, which is also hereby incorporated by reference.
An 802.11 wireless local area network (LAN) is based on a group or cellular architecture where the system is subdivided into basic units or cells. Each cell (called a Basic Service Set, or BSS, in the IEEE 802.11 nomenclature) is controlled by a base station, called an Access Point. Although a wireless LAN may be formed by a single cell, with a single access point, (or no access point), most installations will be formed by several cells, where the access points are connected through some kind of backbone (called Distribution System or DS). This backbone is typically Ethernet, but may be any other type of network, such as a token ring. The whole interconnected wireless LAN, including the different cells, their respective access points and the Distribution System, is seen as a single 802 network to the upper layers of the Open Systems Interconnection model and is known in the Standard as the Extended Service Set (ESS).
An optional power save mode, Power-Save Protocol (PSP), available under the IEEE 802.11 standard that a station can enter or leave, enables the station to conserve battery power when there is no need to send data. With power save mode on, the station indicates its desire to enter a “sleep” state to the access point via a status bit located in the header of each frame. The access point takes note of each station wishing to enter into power save mode and buffers packets corresponding to the sleeping station. In order to still receive data frames, the sleeping station must wake up periodically (at a synchronized time) to receive regular beacon transmissions coming from the access point. These beacons identify whether sleeping stations have frames buffered at the access point. After requesting and receiving the frames, the station can go back to sleep.
Pursuant to the IEEE standard, if a single 802.11 BSS station enters Power-Save Protocol (PSP) operation, the entire BSS adopts different characteristics in order to be able to provide services to the PSP station. Under normal network use (e.g. data latency-tolerant transmission) this PSP operation may not present a hardship. For example, PSP operation would cause a slight lag and delay in the transmission of multicast and broadcast packets, which would not be noticeable in data latency-tolerant transfer environments.
However, for transmissions of more urgency such as low-latency multicast, (e.g. voice-over-internet Protocol (VoIP), video), such a lag and delay time becomes noticeable and problematic to the application.
In a BSS where one of the stations has entered PSP operation, the BSS-wide characteristics will have changed to accommodate the PSP station. For example, in accordance with the IEEE standards relating to PSP operation, when a single BSS station enters PSP mode, all subsequent multicast and broadcast transmissions are queued by the AP and transmitted as a batch following the next 802.11 Data Traffic Indicator Mark (DTIM) beacon. During this transmission time, the access point takes a high priority stance in BSS transmissions, relegating all other transmissions to secondary status, until such time as the DTIM has been completed. In essence, all other traffic in the BSS, both to and from the access point, are effectively halted for the DTIM transmission.
Virtual networking refers to the ability of switches and routers to configure logical topologies on top of the physical network infrastructure allowing any arbitrary collection of LAN segments within a network to be combined into an autonomous station group, appearing as a single LAN. Virtual local area networks (VLANs) offer significant benefits in terms of efficient use of bandwidth, flexibility, performance, and security. VLAN technology functions by logically segmenting the network into different “broadcast domains” whereby packets are only switched between ports that are designated for the same VLAN. Thus, by containing traffic originating on a particular LAN only to other LAN's within the same VLAN, switched virtual networks avoid wasting bandwidth.
The VLAN approach also improves scalability, particularly in LAN environments that support broadcast- or multicast-intensive protocols as well as other applications that flood packets throughout the network.
A problem associated with access points conforming minimally to the 802.11 protocol is that if there is a single station in PSP mode associated to an access point, all multicast frames on all VLANs are buffered and transmitted immediately following an 802.11 DTIM beacon. Accordingly, higher-priority Quality-of-Service unicast transmissions may be delayed for the duration of the multicast delivery period. Power-save stations must stay awake, for the duration of the multicast delivery period, to receive multicast transmissions. As a result, multicast transmissions can reduce battery life in power-save stations. Additionally, stations not in PSP operation and subscribed to low-latency multicast streams must wait for delivery of those multicasts.
In other words, assume station A and station B are both clients of an 802.11 access point. Further, assume that station A enters into 802.11 PSP operation while station B remains in active operation. In the event that station B subscribes to an IP multicast group, the 802.11 access point will buffer the IP multicast stream to compensate for station A being in PSP mode, even though station A is not a subscriber to the IP multicast group. Taking this example further, assume that station A is in VLAN 1 and station B is in VLAN 2, holding all other variables as in the preceding example. Since both station A and station B are clients of the same 802.11 access point, while station A is in PSP mode, transmissions to station B are buffered to compensate for the station A PSP operation. Thus, even though the stations are on different respective VLANs, transmissions must still be buffered because of the PSP mode of station A.
Thus, there exists a need for a system and method which may be suitably configured to immediately transmit low-latency multicasts/broadcasts to VLANs containing only active stations.